One of the most powerful chapters in the book is about evolution. Reichenbach starts by contrasting the inorganic world, which obeys the laws of physics, with the organic world, which is goal directed. But then he goes on to show that the semblance of design and purpose can be accounted for by an evolutionary explanation, and that all biological phenomena can be reduced to physical phenomena. We do not need two separate sciences to account for non-living and living phenomena and can have a unified science about matter. Anticipating synthetic biology, Reichenbach suggests that future science should be able to create life through purposeful manipulation of inorganic matter. Then Reichenbach moves from the evolution of the microworld to the evolution of the universe and reviews how contemporary findings in physics and astronomy affect questions about the past and the future of the universe.

Throughout his discussion of the relationship of science and philosophy, Reichenbach presents a number of distinct logical positivist positions:

It has become a favorite argument of antiscientific philosophies that explanation must stop somewhere, that there remain unanswerable questions. But the questions so referred to are constructed by a misuse of words. Words meaningful in one combination may be meaningless in another. Could there be a father who never had a child? Everyone would ridicule a philosopher who regarded this question as a serious problem. The question of the cause of the first event, or of the cause of the universe as a whole, is not of a better type. The word “cause” denotes a relation between two things and is inapplicable if only one thing is concerned. The universe as a whole has no cause, since, by definition, there is no thing outside of it that could be its cause. Questions of this type are empty verbalisms rather than philosophical arguments.

At the end of the chapter, Reichenbach criticizes the widespread view that there are other means of establishing knowledge which can answer questions that science cannot:

The elimination of meaningless questions from philosophy is difficult because there exists a certain type of mentality that aspires to find unanswerable questions. The desire to prove that science is of a limited power, that its ultimate foundations depend on faith rather than on knowledge, is explainable in terms of psychology and education, but finds no support in logic. There are scientists who are proud of when their lectures on evolution conclude with a so-called proof that there remain questions unanswerable for the scientist. The testimony of such men is often invoked as evidence for the insufficiency of a scientific philosophy. Yet it proves merely that scientific training does not always equip the scientist with a backbone to withstand the appeal of a philosophy that calls for submission to faith. He who searches for truth must not appease his urge by giving himself up to the narcotic of belief. Science is its own master and recognizes no authority beyond its confines.

This passage raises the important question of whether the position of logical empiricism is self-applicable. The same issue has been encountered by critical rationalists. One “solution” to this challenge is to make critical rationalism coherent by holding all positions open to criticism, including critical rationalism itself. This approach, called “pancritical rationalism” or “comprehensive critical rationalism,” has been proposed by the philosopher William Warren Bartley in his book The Retreat to Commitment. Bartley’s solution has been criticized for producing logical paradoxes and its vacuous nature. Hans Reichenbach response was to develop a distinct probabilistic account of knowledge to avoid some of the remaining “rationalist” tendencies in contemporary empiricism.

Logical positivism found itself in the peculiar situation of struggling with its own internal consistency while at the same time seeing many of its basic tenets reflected in contemporary scientific practice. One of Hans Reichenbach’s projects was to develop a scientific account of philosophy to resolve this situation.